The invention relates to a reservoir tank for a reducing agent, in particular an aqueous urea solution for the after-treatment of exhaust gas from a combustion engine, comprising an outer container and a pot-shaped inner container which limits a partial volume of the volume of the outer container, a heating element which is set in the inner container and an extraction device for extracting the reducing agent.
Due to steadily growing statutory requirements for the emission values of combustion engines, the exhaust gas from combustion engines is subjected to an after-treatment in order to comply with the predefined limits. In order to reduce the nitrogen oxide emissions, particularly in diesel engines, reduction catalysts are used by means of which the nitrogen oxide emissions (NOx) can be significantly lowered. Before the exhaust gas reaches the catalytic converter, a reducing agent is fed into said exhaust gas, as, for example, an aqueous urea solution. Said solution causes the formation of ammonia, which in turn reacts with the nitrogen oxides in the downstream catalytic converter to form harmless nitrogen and water. The supply of the aqueous urea solution takes place via metering systems, which as a rule comprise a reservoir tank for storing the reducing agent and a metering module as, for example, a metering pump or a metering valve. Such a metering system further requires a delivery module in order to supply the metering module with the reducing agent stored in the reservoir tank.
The reservoir tank provided for storing the reducing agent is normally disposed in the vehicle in such a way that the reducing agent contained therein is exposed to the outside temperatures. Because the aqueous urea solution freezes at temperatures below −11° C., the reservoir tank is equipped with a heating element, which prevents said solution from freezing or causes already frozen reducing agent to thaw. The heating element can, for example, relate to an electric heater. Self regulating PTC heating elements (Positive Temperature Coefficient), which are integrated into a flat carrier, can be used, for example, particularly advantageously at high temperatures as heating elements. Regardless of the precise embodiment of the heating element, said heating element is preferably disposed in the floor region of a pot-shaped container that is set in the tank, said container ensuring that said heating element is surrounded by a sufficient quantity of reducing agent even at a low fill level.
A reservoir tank of the aforementioned kind is known for example from the German patent application DE 10 2006 046 899 A1. The reservoir tank comprises an outer container, an inner container as well as a heating element, which is accommodated in the inner container. The reservoir tank further comprises an extraction device, with which the liquid reducing agent can be extracted from the tank. The inner container is thereby connected to the outer container in such a way that liquid reducing agent can flow out of said inner container into said outer container. The heating element, which is accommodated in said inner container, causes frozen reducing agent to thaw at low ambient air temperatures, which due to the fluidic connection between the two containers travels into said outer container and thereby causes the still frozen reducing agent contained therein to thaw. In order to fluidically connect said inner container to said outer container, the invention further proposes for said inner container to be provided with at least one aperture through which fluid can flow out of said outer container into said inner container and vice versa.
Because the frozen reducing agent has in comparison to the liquid reducing agent a considerably higher thermal conductivity, the liquid reducing agent leaving the inner container via the apertures causes the heat in the volume of ice in the outer container to be distributed or respectively reduced so that the heating element disposed in said inner container is only capable of thawing the frozen reducing agent present in said inner container. This result has a very critical effect on the motor vehicle's operation if the fill level is very low and/or the tank is very flat. The case can then arise where a sufficient quantity of liquid reducing agent is not available for the required exhaust gas after-treatment. The vehicle possibly has to then be shut down.
The aim of the invention is therefore to provide a reservoir tank for reducing agent of the kind previously mentioned, which facilitates an efficiency-optimized use of the heating element. The proposed reservoir tank shall additionally be able to have a flat construction.